A Multiple Input Multiple Output (MIMO) system refers to a system for improving data transmission/reception efficiency using multiple Transmission (Tx) antennas and multiple Reception (Rx) antennas. In a MIMO system, each transmission antenna has an independent data channel. The Tx may be a virtual antenna or a physical antenna. A receiver estimates a channel with respect to each Tx antenna and receives data transmitted from each Tx antenna based on the channel estimation.
Channel estimation refers to a process of compensating for signal distortion caused by fading so as to restore the received signal. Fading refers to a phenomenon in which the intensity of a signal is rapidly changed due to multi-path time delay in a wireless communication system environment. For channel estimation, a reference signal known to both a transmitter and a receiver is necessary. The reference signal may be abbreviated to RS or referred to as a pilot signal according to the standard.
A downlink RS is a pilot signal for coherent demodulation, such as a Physical Downlink Shared Channel (PDSCH), a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid Indicator Channel (PHICH), and a Physical Downlink Control Channel (PDCCH). The downlink RS includes a Common Reference Signal (CRS) shared among all UEs in a cell and a Dedicated Reference Signal (DRS) for a specific UE.
In a system having an antenna configuration (e.g., a system according to the Long Term Evolution-Advanced (LTE-A) standard supporting eight Tx antennas) developed as an extension of a legacy communication system (e.g., a system based on LTE Release 8 or 9) supporting four Tx antennas, DRS-based data demodulation has been considered in order to support efficient RS management and develop an advanced transmission scheme. That is, in order to support data transmission through extended antennas, DRSs for two or more layers may be defined. Since the DRSs are precoded using the same precoder as used for data, it is possible for a receiver to easily estimate channel information for demodulating data without separate precoding information.
A downlink receiver may acquire precoded channel information with respect to the extended antenna configuration through DRSs. However, in order to acquire non-precoded channel information, separate RSs are required in addition to the DRSs. In a system based on the LTE-A standard, RSs for acquiring Channel State Information (CSI) at a receiver, that is, CSI-RSs, may be defined.